As is known, mixing trucks are machines used in the agricultural field, particularly in animal farms, for mixing forage and optionally reducing its dimensions and volume.
These trucks are constituted substantially by a body inside which there are screw feeders, generally two contrarotating screw feeders, which are turned by a transmission unit, which draws its motion from a power take-off of a farming tractor by means of a cardan shaft or the like.
The screw feeders can be provided with blades which shred the forage in addition to mixing it.
Among mixing trucks, desiling and mixing trucks are known in particular which in addition to mixing and optionally shredding the forage also remove it from storage silos and load it into the body of the truck (desiling) by means of a loading mill.
Desiling occurs generally while the screw feeders are moving; there is in fact a permanent functional connection, so that the actuation of the loading mill entails the automatic actuation of the screw feeders.
In order to separate the actuation of the screw feeders from the actuation of the loading mill or of any other auxiliary user devices, reduction units are known which are the subject of Italian industrial invention patent application No. MI2001A00436 and of the corresponding European patent EP1,236,606 in the name of the same Applicant and are substantially constituted by an off-center gearbox, which can be associated with the power take-off of a tractor and forms couplings for connection to auxiliary user devices, by a reduction unit which is functionally collected to the off-center gearbox, and by a friction clutch, which is interposed between the off-center gearbox and the reduction unit and allows to interrupt the transmission of motion to the screw feeders but not to the auxiliary user devices.
However, these known reduction units are susceptible of further improvements.
In particular, it is noted that if the truck jams as a consequence of an excessive accumulation of forage between the screw feeders which blocks them, it is necessary to stop its operation and reverse the direction of rotation of the screw feeders in order to remove the forage that is stuck between them.
In known mixing trucks, such unblocking operation is performed by disconnecting manually the power take-off from the main input shaft of the reduction unit and connecting it to a shaft that is adjacent thereto, so as to reverse the direction of rotation in output from the reduction unit.
This operation entails performing particularly time-consuming and labor-intensive maneuvers, which slow the preparation of the fodder and by being performed manually endanger the safety of the operators.
Further, this operation entails the need to provide in the reduction unit an auxiliary input, which is constructively complicated and entails substantial bulk, which can interfere with the devices for protecting the cardan shaft or with the auxiliary user devices connected to the off-center gearbox; moreover, such user devices must be of the bidirectional type.
As an alternative, if the tractor is provided with a power take-off that can be synchronized with the reverse gear, it is possible to reverse the direction of rotation of the screw feeders by driving the truck backward, a maneuver which in itself is not advisable for safety reasons.
In reduction units provided with a friction clutch, the rotation rate generated in reverse gear and transmitted to the auxiliary user devices, particularly to the pump of the hydraulic circuit that drives the coupling of the clutch device, can be disadvantageously lower than the rotation rate required to generate the necessary pressure for the coupling of the device; accordingly, the screw feeders remain motionless and the truck remains jammed. Finally, it is noted that the operations for detecting the jamming of the truck and the release operations are performed and controlled manually by the assigned operators; therefore, they are generally performed late and/or for excessively long times, subjecting the screw feeders and the rotary motion transmission elements to overloads and stresses which, over time, compromise their integrity and correct operation.